A number of foreign hostages at a besieged Algerian gas plant have reportedly been freed in the wake of a military strike which news agencies say has killed several people.

Mauritania's ANI news agency reported the deaths citing one of the kidnappers who had earlier taken 41 people, including Britons, captive at the compound near the border with Libya.

And French president Francois Hollande said the hostage crisis was unfolding in "terrible conditions".

The official Algerian APS news agency said a number of foreign hostages were free. Among them was 36-year-old Irishman Stephen McFaul from west Belfast.

His father, Christopher McFaul, told Sky News he was "delighted" his son was no longer being held captive but added he felt "sorry for the other hostages that are still there". He also described the last 48 hours as "hell".

Stephen McFaul's son, Dylan, said: "I can't even explain the excitement. I can't wait until he gets home again."

He added he would tell him "he's never going back there and I'm not letting him".

APS said as many as 600 Algerian workers at the site managed to flee the plant, and an American military official said at least one US unarmed drone was flying over the complex.

A large explosion has been heard away from the accommodation block at the remote desert plant, Sky sources said.

News agency Reuters spoke to a local resident who said many people were killed in the military operation, which the Algerian government confirmed it was carrying out.The gas complex is at In Amenas, in eastern Algeria

The Algerian government said it had to act "immediately" to intervene in the hostage crisis, according to Downing Street.

David Cameron's spokesman said Britain was not given prior notice of the operation, adding it was ongoing and the situation was "very grave and serious".

Mr Cameron was informed the operation was under way when he telephoned the Algerian prime minister at 11am today, the spokesman added.

And the British PM made clear he would have preferred to be informed in advance of the military action, the spokesman went on.

There have been conflicting reports as to the number of casualties, with Reuters citing a local source as saying six foreign hostages and eight rebels were killed.

The source said some hostages were still being held.

ANI said the attackers' spokesman had claimed that Algerian forces opened fire as the militants tried to leave the energy complex with hostages.

The facility is run jointly by BP, the Norwegian energy firm Statoil and the Algerian state oil company.

On Wednesday a Briton and an Algerian were killed when around 20 gunmen from an al Qaeda-backed group stormed the In Amenas facility.

Among the UK nationals taken hostage by the group were Scottish nationals, Scottish National Party leader Alex Salmond confirmed earlier.